How do we know that Christ's kingdom is spiritual?

In John 18:36, Jesus declares, 'My kingdom is not of this world.' This remark underscores that the intent of Christ's ministry is not to establish an earthly or political regime but to bring forth a spiritual reality. The expectations of the disciples and others at the time were for a Messiah who would overthrow Roman rule and restore Israel's political sovereignty. Jesus, however, focused on inaugurating a spiritual kingdom established by the regeneration of believers through the Holy Spirit. His kingdom is characterized by righteousness, peace, and joy in the Holy Spirit (Romans 14:17). This truth is foundational in sovereign grace theology, showing that God’s redemptive plan focuses on transforming hearts rather than exerting temporal power.